With the advancement of communication infrastructure in portable electronic devices, particularly, in portable phones, more information (image information such as characters, graphics, illustrations, and photographs) are communicated at higher speed. In order to display these information, there is demand for a liquid crystal display section which makes up a display section of portable electronic devices to have a higher display quality with higher resolutions.
To increase resolutions in the liquid crystal display area means increase in number of dots, i.e., pixels, which results in increase in power consumption of the portable electronic devices. Meanwhile, a total power consumption of portable electronic devices needs to be low in order to extend the life of batteries making up a power source.
In order to meet this demand, there have been proposals to reduce power consumption by way of displaying only a required portion of the liquid crystal display section as an image display area.
Conventionally, partial display driving of a TFT (Thin Film Transistor) liquid crystal panel, which is an active-matrix liquid crystal display section, was performed in such a way that a non-image area and an image display area were driven at the same timing. Further, in the case of a simple-matrix liquid crystal panel, Japanese Unexamined Patent Publication No. 184434/1999 (Tokukaihei 11-184434) (published date: Jul. 9, 1999) discloses applying a white signal voltage in the non-image area by applying means prior to a transition into a partial display state. This publication partially recites partial display in a scanning direction of the active-matrix type.
However, in the foregoing publication, the applied voltage gradually changes, i.e., decreases in the pixels in the non-image area to which the white signal voltage was applied, and therefore it was required to apply another white signal voltage to maintain white display. That is, the foregoing publication discloses applying a voltage, equivalent to the voltage applied to the counter electrodes, once to the non-image area, and no subsequent application of the voltage is made to the non-image area (non-display portion). However, since the applied charge diminishes over time in the active-matrix liquid crystal panels, this method is not applicable to the active-matrix liquid crystal panels and it is required to apply a voltage for a certain period also in the non-image area. Thus, the foregoing publication failed to achieve lower power consumption, due to the application of a new white signal voltage.
Further, in the conventional method, in order to maintain the applied voltage, it was required in the active-matrix liquid crystal display panel having counter electrodes, when performing partial display in the scanning line direction, to apply a white signal voltage of the opposite polarity also to a non-display portion where the partial display in the scanning line direction is performed, so as to avoid such drawbacks as image persistence of the liquid crystal.
Conventionally, the non-display portion was scanned by the count-up of the shift register of the gate driver per one horizontal period in the same manner as the display portion. In this case, the output of video signals from the source driver needs to be created, apparently, for the number of outputs of the entire scanning lines, and the power consumption of the liquid crystal panel for the partial display becomes equivalent to that of entire display, thus failing to achieve lower power consumption.
Note that, Japanese Examined Patent Publication No. 2585463/1996 (published date: Jun. 11, 1992) discloses realizing display without change in time axis, when the number of scanning lines in the display area is larger than that of the effective scanning lines of the input video signals, by simultaneously scanning a plurality of scanning lines, other than the scanning lines of the effective display portion, in a retrace period within one frame period.
However, in the method as disclosed in the foregoing publication No. 2585463/1996, for example, when the effective display portion is positioned on the bottom of the display area (display screen), all areas are scanned normally and it fails to solve the foregoing problems. Further, while this publication teaches simultaneously scanning a plurality of scanning lines, other than the scanning lines in the effective display area, its purpose is to simplify the circuits when the number of horizontal lines in a vertical period (the number of horizontal counts in a vertical period) is smaller than the number of scanning lines of the display device, and it is not for realizing lower power consumption, and, in fact, the publication is silent as to operation for realizing lower power consumption. Thus, lower power consumption is not achieved. Further, the publication does not take into consideration the case where the number of horizontal lines in a vertical period (the number of horizontal counts in a vertical period) is larger than the number of scanning lines in the display device. Further, the foregoing publication is totally silent as to preventing flicker on the screen.